CHAP. IL] RESPIRATION. 581 



and expired air arises from the fact that the volume of any given 

 quantity of carbonic acid is equal to the volume of the oxygen 

 consumed to produce it ; the slight falling short of the expired air 

 is due to the circumstance that all the oxygen inspired does not 

 reappear in the carbonic acid expired, some having formed within 

 the body other combinations. 



341. Besides carbonic acid, expired air contains various 

 substances which may be spoken of as impurities, many of an 

 unknown nature, and all in small amounts. Traces of ammonia 

 have been detected in expired air, even in that taken directly 

 from the trachea, in which case its presence could not be due 

 to decomposing food lingering in the mouth. When the expired 

 air is condensed by being conveyed into a cooled receiver, the 

 aqueous product is found to contain organic matter, which, from 

 the presence of micro-organisms, introduced in the inspired air, 

 is very apt rapidly to putrefy. The organic substances thus 

 shewn to be present in the expired air are the cause in part of 

 the odour of the breath. Although the matter has been disputed 

 it is probable that some of these substances are of a poisonous 

 nature, either poisonous in themselves as coming direct from and 

 produced in some way or other in the pulmonary apparatus, or 

 poisonous as being the products of putrefactive decomposition; 

 for various animal substances and fluids give rise by decomposition 

 to distinct poisonous products, known as ptomaines, and it is 

 possible that some of the constituents of expired air are of an 

 allied nature. The presence of these substances probably explains 

 why, although an atmosphere containing simply 1 p.c. of carbonic 

 acid (with a corresponding diminution of oxygen) has very little 

 effect on the animal economy, an atmosphere in which the car- 

 bonic acid has been raised to 1 p.c. by breathing, is highly in- 

 jurious. In fact, air rendered so far impure by breathing (to be 

 exact we ought perhaps to say " living in," since something may be 

 given off by the skin,) that the carbonic acid amounts to '08 p.c. is 

 distinctly unwholesome, not so much on account of the carbonic 

 acid, as of the accompanying impurities. Since these impurities are 

 of unknown nature and cannot be estimated, the easily determined 

 carbonic acid is usually taken as an indirect measure of their 

 presence. We have seen that the average man loads, at each 

 breath, 500 c.c. of air with carbonic acid to the extent of 4 p.c. He 

 will accordingly at each breath load 2 litres to the extent of 1 p.c. ; 

 and in one hour, if he breathe 17 times a minute, will load rather 

 more than 2000 litres to the same extent. At the very least then 

 a man ought to be supplied with this quantity of air hourly ; and 

 if the air is to be kept fairly wholesome, that is with the carbonic 

 acid reduced considerably below *1 p.c., he should have even more 

 than ten times as much. 



